Chiral symmetry breaking dictated by electric-field-driven shape transitions of nucleating conglomerate domains in a bent-core liquid crystal

Generating and controlling chiral symmetry breaking and enantiomeric excess is not only interesting from a fundamental perspective but can also lead to novel functional materials. In this work, we show how the dark conglomerate (DC) liquid crystalline phase characterized by macroscopic chiral domains offers such a possibility if formed under an electric field. In addition the chiral domains are electro-optically switchable. The chiral segregation in the DC phase can be tuned by using dc or ac fields at different frequencies. Consequently, the enantioselectivity, dielectric parameters and switching polarization in the DC phase become tunable. Another interesting aspect is that the nucleating conglomerate domains formed under ac fields exhibit frequency dependent shape transitions which have a striking resemblance to domain shape changes observed in two-dimensional monolayers. This can therefore be used as a model experimental system to get a physical insight into the effects of chiral and electrostatic interactions, under external fields, on domain growth and interface structures. The domain shape transitions can also be used to investigate the role of growth morphology in coarsening and scaling hypotheses. From a technological point of view this opens up the possibility of obtaining chiral thin films with preferential sense of chirality which can be useful in chiroptical and nonlinear optical applications.

Figure 1

Formation of the DC${}_{\mathrm{dc}}$ phase from the isotropic phase under a dc field of 10 V/μm in a sample cooled at 1 °C/min. (a) Conglomerate fractal domains (126.6 °C), (b) further growth of fractal domains (126.3 °C), (c) and (d) well formed chiral domains (125 °C). Analyzer rotated by 2° in the clockwise direction from the crossed position in (a)–(c) and anticlockwise direction in (d).

Figure 2

Formation and racemization of the DC${}_{\mathrm{ac}}$ phase under a square wave ac field of 10 V/μm, $f$ = 100 Hz in a sample cooled at 1 °C/min. (a) Chiral domains (128 °C), (b) formation of small domains of opposite chirality within the larger domains on switching off the ac field (124 °C), (c) onset of racemization (120 °C). Analyzer rotated by 2° from the crossed position in the clockwise direction in all cases.

Figure 3

Formation of the DC${}_{\mathrm{ac}}$ phase from the isotropic phase under a square wave ac field of 10 V/μm, in a sample cooled at 1 °C/min with the nucleating conglomerate domains undergoing shape transitions as a function of frequency. (a) Circular domains (124 °C) and (b) corresponding DC${}_{\mathrm{ac}}$ phase (116 °C) obtained when $f$ = 2 kHz; (c) domains with undulating boundaries (124 °C) and (d) corresponding DC${}_{\mathrm{ac}}$ phase (116 °C) obtained when $f$ = 3 kHz; (e) branched domain structures (124 °C) and (f) corresponding DC${}_{\mathrm{ac}}$ phase (116 °C) formed when $f$ = 4 kHz, analyzer rotated by 2° from the crossed position in the clockwise direction in all cases.

Figure 4

Comparison of the histograms generated from pixel intensity values obtained from the optical textures with chiral domains in the DC${}_{\mathrm{dc}}$ and DC${}_{\mathrm{ac}}$ phases.

Figure 5

Electro-optic switching in the DC${}_{\mathrm{dc}}$ phase under a triangular wave field at 121 °C, 10 V/μm at $f$ = 10 Hz. (a) and (b) Optical textures showing the interchange of the chiral sense in the domains on reversal of field. Analyzer rotated by 2° from crossed position in the clockwise direction in both cases. (c) Corresponding switching current response.

Figure 6

Temperature dependence of the switching polarization in the DC${}_{\mathrm{dc}}$ and DC${}_{\mathrm{ac}}$ phases.

Figure 7

(a) Temperature variation of the real part of the dielectric permittivity (${\varepsilon }^{\prime }$) in the DC${}_{\mathrm{dc}}$ and DC${}_{\mathrm{ac}}$ phases measured at $f$ = 1 kHz. (b) Frequency variation of the imaginary part of the dielectric permittivity (${\varepsilon }^{\prime \prime }$) in the DC${}_{\mathrm{dc}}$ and DC${}_{\mathrm{ac}}$ phases at 116 °C.

Figure 8

Images used to calculate the fractal dimension of the domains obtained under a dc field of 10 V/μm. (a) Optical texture of the fractal domain, (b) binary image used for the fractal box counting method, (c) plot of number of boxes with increasing size against box size used to obtain the fractal dimension $D$ by imagej software.